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Author Topic:   Classic 13, 50-HP

13rounds posted 07-20-2007 09:58 AM ET (US)
I have a 13-foot Boston Whaler classic with a Johnson 50. Should I use a 12.25 x 15 pitch propeller or a 11.75 x 17 pitch propeller?
Tom W Clark posted 07-20-2007 10:01 AM ET (US)
I suggest you tell us what year your Johnson 50 is, what the gear ratio is and what the recommended WOT range of it is so reasoned advice can be offered.
13rounds posted 07-20-2007 10:04 AM ET (US)
[I] have no idea what the gear ratio is. [T]he motor is a 1991 but [I] seem to be hitting the rev limiter with the propeller [I] have now. [I] don't know if it's because the propeller is worn out--[I] mean it is really old.
CaMoFroG posted 07-20-2007 10:19 AM ET (US)
Is your current prop a 14" pitch ?
I am pretty sure rule of thumb is every 2" increase in pitch is 400 rpm decrease [and] vise versa. If you decrease the pitch 2" it will increase the rpm 400.
more pitch = more top end speed less torque
less pitch = more torque (better hole-shot) less top end speed
13rounds posted 07-20-2007 10:26 AM ET (US)
Thanks Camo, right now I have a 15 pitch and I'm hitting the rev limiter. I'm not too concerned about having less torque because as of right now the boat cavitates from having too much torque at take off. Should I go with aluminum or stainless and where's the cheapest place?
Tohsgib posted 07-20-2007 10:51 AM ET (US)
I am running a 11.75x17 stainless with my 40 and can hit redline but I also have a jackplate. I have a 17" alum if you are interested. Do you have a tach? I believe your redline is 5500 but double check.
13rounds posted 07-20-2007 10:53 AM ET (US)
I'm looking for a stainless since I will be running in shallow water. No I don't have a tachometer, I was going to install one but my mechanic said that with these little motors the tachometers weren't very accurate.
Tohsgib posted 07-20-2007 11:40 AM ET (US)
Your mechanic is crazy...plus they are much more accurate than no tach at all. Hiting a rev limiter will DRASTICALLY reduce engine life. Mainly because redline is say 5500 but rev limiter does not kick in until say 6k.
13rounds posted 07-20-2007 11:47 AM ET (US)
[I]n that case [I]'ll be buying a tachometer today and installing it myself. [W]hat brand is your stainless prop?
jimh posted 07-21-2007 09:20 AM ET (US)
The electrical operation of a tachometer is fundamentally identical whether the motor has 50-HP of 250-HP, so there is no basis to suggest that there is a difference in accuracy based on horsepower.
I am quite surprised that the engine has a rev limiter in it. How did you determine it has a rev limiter?
13rounds posted 07-21-2007 07:00 PM ET (US)
the revs just kept bouncing up and down. thank you all for your suggestions i purchased an 11.75x17 today and the boat is running a million times better and the motor ran great!

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Author	Topic: Classic 13, 50-HP
13rounds	posted 07-20-2007 09:58 AM ET (US) I have a 13-foot Boston Whaler classic with a Johnson 50. Should I use a 12.25 x 15 pitch propeller or a 11.75 x 17 pitch propeller?
Tom W Clark	posted 07-20-2007 10:01 AM ET (US) I suggest you tell us what year your Johnson 50 is, what the gear ratio is and what the recommended WOT range of it is so reasoned advice can be offered.
13rounds	posted 07-20-2007 10:04 AM ET (US) [I] have no idea what the gear ratio is. [T]he motor is a 1991 but [I] seem to be hitting the rev limiter with the propeller [I] have now. [I] don't know if it's because the propeller is worn out--[I] mean it is really old.
CaMoFroG	posted 07-20-2007 10:19 AM ET (US) Is your current prop a 14" pitch ? I am pretty sure rule of thumb is every 2" increase in pitch is 400 rpm decrease [and] vise versa. If you decrease the pitch 2" it will increase the rpm 400. more pitch = more top end speed less torque less pitch = more torque (better hole-shot) less top end speed
13rounds	posted 07-20-2007 10:26 AM ET (US) Thanks Camo, right now I have a 15 pitch and I'm hitting the rev limiter. I'm not too concerned about having less torque because as of right now the boat cavitates from having too much torque at take off. Should I go with aluminum or stainless and where's the cheapest place?
Tohsgib	posted 07-20-2007 10:51 AM ET (US) I am running a 11.75x17 stainless with my 40 and can hit redline but I also have a jackplate. I have a 17" alum if you are interested. Do you have a tach? I believe your redline is 5500 but double check.
13rounds	posted 07-20-2007 10:53 AM ET (US) I'm looking for a stainless since I will be running in shallow water. No I don't have a tachometer, I was going to install one but my mechanic said that with these little motors the tachometers weren't very accurate.
Tohsgib	posted 07-20-2007 11:40 AM ET (US) Your mechanic is crazy...plus they are much more accurate than no tach at all. Hiting a rev limiter will DRASTICALLY reduce engine life. Mainly because redline is say 5500 but rev limiter does not kick in until say 6k.
13rounds	posted 07-20-2007 11:47 AM ET (US) [I]n that case [I]'ll be buying a tachometer today and installing it myself. [W]hat brand is your stainless prop?
jimh	posted 07-21-2007 09:20 AM ET (US) The electrical operation of a tachometer is fundamentally identical whether the motor has 50-HP of 250-HP, so there is no basis to suggest that there is a difference in accuracy based on horsepower. I am quite surprised that the engine has a rev limiter in it. How did you determine it has a rev limiter?
13rounds	posted 07-21-2007 07:00 PM ET (US) the revs just kept bouncing up and down. thank you all for your suggestions i purchased an 11.75x17 today and the boat is running a million times better and the motor ran great!